Currently on the market there are syringe infusion pumps that hold drug filled syringes and empty the contents into patients requiring intravenous infusions. Most typically, syringe infusion pump drives are motor driven leadscrew assemblies. Also, for easy resetting of the drive when an emptied syringe is to be replaced with a filled syringe, a halfnut driving nut is typically used. For accurate drug delivery, it is necessary that the drive be fully engaged without slipping under load. Nonetheless, for ease of use by nursing personnel, the drive must be easily decoupled for resetting without excessive force or unusual skill.
Halfnut/leadscrew drive mechanisms for syringe infusion pumps have therefore been used to translate rotary motion to linear motion yet provide convenient drive disengagement if required. Because of their ability to be disengaged by the operator, their design is not necessarily straightforward. They need to be easily disengaged, yet must remain fully engaged under all expected driving conditions.
A guided halfnut of the type disclosed in U.S. Pat. No. 4,544,369 requires a relatively large supplemental engagement force between the halfnut and leadscrew to assure engagement under all conditions. This force is typically supplied by a spring. This spring force must be overcome by the operator during disengagement. Further, the necessary spring force increases with increasing drive loads. Stated another way, the drive load will bring about forces which tend to separate the leadscrew and halfnut, and these forces must be counteracted by an external, supplemental force.
The previously disclosed commercial infusion pumps used a spring loaded halfnut of similar design that was guided with respect to the leadscrew by other components in the assembly. A lever or button was squeezed to effect decoupling. This arrangement required a force of over 4 lbs. to assure that the halfnut remained engaged. This high force proved to be difficult to overcome by nurses. This resulted not only in inconvenience, but also inadvertent product abuse, since the halfnut was often scraped against the leadscrew as the drive assembly was reset.
A jamming halfnut has also been disclosed in the prior art and can be designed such that the drive load causes forces which tend to engage the halfnut with the leadscrew. The geometry of the system is made so that the halfnut/leadscrew reaction and the halfnut/pusher block reaction are equal but opposite Therefore, the separation component of the former is countered by the restoring component of the latter and engagement is maintained.
It was clear that lowering the coupling spring force would improve the product by making it easier to decouple The problem overcome by this invention was how to accomplish this without compromising, and, hopefully, while even improving the drive system's life and drive capability.
Several potential solutions were considered and dismissed. Mechanisms with mechanical advantage could lower the disengagement force required, but needed additional parts and therefore assembly complexity. A pivoting halfnut was very inviting because the halfnut had a self-jamming effect. That is, the drive force resulted in a directly proportional coupling force. Because of this, the spring force could be made virtually zero if the pivot point was properly chosen. It was discovered, however, that while the drive held firmly against the normal drive forces, the drive assembly could too easily be advanced forward over the leadscrew threads if the loads were reversed. This could result in a dangerous drug overdose if the drive assembly was bumped or hit. Further, the drive assembly again required additional complexity.